Low density lipoprotein (LDL) is the molecular control of cholesterol synthesis and cholesterol ester formation in cells: thus it is probably directly involved in the processes of cellular cholesterol accumulation and atherogenesis. However, only a limited knowledge of the characteristics of the protein of LDL, the B protein, is available. Possible peptide heterogeneity of the B proteins of LDL and VLDL has been demonstrated in the presence of sodium dodecyl sulfate. Separation and characterization of the B proteins of VLDL and LDL will be accomplished using techniques of gel filtration, preparative polyacrylamide gel electrophoresis, amino acid analysis, and peptide mapping. Phenomena of association and dissociation of the major peptides of the B proteins will be studied by gel filtration in dissociating systems, e.g., guanidine HCl, chaotropic salts, and SDS containing buffers. The minimum molecular weight of the major B proteins of LDL will be determined by quantitative amino acid analysis of the separated products of cyanogen bromide cleavage. An attempt will be made to correlate LDL genotypes with specific peptide heterogeneity of LDL.